Currently SQ and MAS operate a duopoly: there are NO LCCs operating between SIN and KUL. SGD414 sounds about right.
There is a 'shuttle' service, you turn up at the airport and queue, I think it works out about $300 return but I have never tried it.
HA is right about the coaches, they are an option but it's a min of 5 hours, and because customs coming back into SGP are a pain, you can spend up to an hour there while they unload the coach and luggage and check everything - their point (I assume) being that cigarette and mucky DVD smugglers more likely to take a cheaper transport option so they need to be more thorough.
Go to BKK or CGK instead with Tiger or Valuair.